1 SLAng Semantics, and Service Composition James Skene, Davide Lamanna, Wolfgang Emmerich
Dec 17, 2015
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Approach to defining the Semantics of SLAng
1. Model the environment of SLAs, i.e. Internet services
2. Model the SLA language itself
3. Embed the language model in the Internet service model
4. Show how SLAs constrain the services (assuming they are respected)
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Embedding SLAs
App
AS
ASPSLA
Hosting
SLANetwork
SLA
Network
SLA
Storage
SLA
Comms
SLA
App
AS
ASPSLA
Hosting
SLANetwork
SLA
Network
SLA
Storage
SLA
ASPSLA
Cont.
SLA
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Features of ASP contract
• Backup clauses:– How frequent
– What type of backup
– Should the backup be encrypted?
– What datatypes
– How can the client access the backups
• Monitoring clauses:– What monitoring solution?
– How frequent
– On demand?
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Features of ASP contract (2)
• Server Performance– Maximum latency for service operations
– Reliability (proportion of time that service should be available)
– Maximum time to repair
• Client Performance– Maximum throughput
• These clauses are scheduled to allow different performance at different times.
• Also the specification of maintenance periods.
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Performance and Reliability
• Expressed using a single constraint:– Proportion of downtime observed to total time that the clause applies must
not be greater than the reciprocal of the reliability.
• Intent:– Any service usage that takes too long or otherwise fails contributes to the
downtime. See enough of these and you can claim unreliability.
• In OCL:context ServerPerformanceClause inv:
self.operation->forAll(o |
totalDowntime(o) < (applicationTime * (1 - reliability)))
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OCL Definitions
context ServerPerformanceClausedef:
-- Returns the client performance clauses governing the performance of -- operation o at time t. let applicableClientClauses(t : double, o : Operation) = sla.clientResponsibilities.clientPerformanceClause->select(c | c.schedule->exists(s | s.applies(t)))
-- An expression for the maximum throughput with which the client can use an -- operation at time t, or -1 if there is no limit let minThroughput(t : double, o : Operation) = if applicableClientClauses->isEmpty() then -1 else applicableClientClauses->iterate( c : ClientPerformanceClause, minTP : double | minTP.min(c.maxThroughput))
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More OCL definitions
-- Amount of downtime observed for a failure at time t. This is 1 / the most
-- restrictive throughput constraint applicable at the time
let downtime(t : double, o : Operation) : double =
if minThroughput(t, o) <= 0 then 0
else 1 / minThroughput(t, o)
-- Total amount of downtime observed for the operation
let totalDowntime(o : Operation) : double =
o.serviceUsage->select(u |
(u.failed or u.duration > maximumLatency) and
schedule->exists(s | s.active(u.date))
)->collect(u | downtime(u.date))->iterate(
p : double, sumP : double | sumP + p)
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Still more OCL definitions!context ScheduledClause def:
-- Next time that the current performance clause applies let nextApplies(t : double) = schedule->collect(s.nextActive(t))->iterate(n : double, minN : double | minN.min(n))
-- One of the next schedules that will apply let nextSchedule(t : double) = schedule->one(s | s.nextApplies(t) = self.nextApplies(t))
-- The remaining time for this schedule to apply after time t -- (recursively adds up period of application of next schedule) let timeRemaining(t : double) = if(nextApplies(t) = -1) then 0 else nextSchedule(t).nextDuration(t) + timeRemaining(nextApplies(t) + nextSchedule(t).nextDuration(t))
-- Total amount of time that this clause will apply let applicationTime = timeRemaining(nextApplies(-1))
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No more after this – I promise
context Schedule def:
-- Calculates t modulo p using operations supported by OCL v.1
let mod(t : double, p : double) =
(t/p - (t/p).floor()) * p
-- Returns true if the schedule applies at time t
let applies(t : double) : boolean =
t > startDate and
mod(t - startDate, period) < duration
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Using the Semantics: Two Notions of Composition
Inter-service composition:How does an offered level of QoS comparewith our requirements?
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Using the Semantics: Two Notions of Composition
Intra-service composition:How do the service components contributeto the QoS of the composed service?
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SLA Compatibility
• Compare offered SLA with requirements expressed in as SLA.
• SLAs impose constraints on the services that they are associated with.
• SLA B is compatible with SLA A if they constrain the same services and B allows no behaviours/configurations that would violate A.
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Implications of our Definition of Compatibility
• Must compare complete SLAs because of interactions between terms– Example: Reliability constraints depends on three
specifications:• maximumLatency• reliability• maximumThroughput – A client constraint
• In this example only SLAs with the same or stronger throughput constraint have the chance to be compatible.
• Stronger throughput constraint may not be acceptable to the client.
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Intra-service Composition
• We need to know what the service is doing. This requires a view of the design of the system
• In general, we need to perform analysis.• Our approach to this is to analyse UML
designs as discussed in previous presentations.
• To integrate SLAs we need notation for SLAs in designs.
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The QoS Profile• ‘UML Profile for Modelling Quality of Service and Fault
Tolerance Characteristics and Mechanisms’, or the QoS profile.
• The QoS profile allows Modelling of QoS Properties.• Profiles are defined with reference to semantic models.
QoSprofile QoS model
Somecharacteristics
Somesystem model
SLAngSemantic
model
SLAngcharacteristics
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Implementing SLAng Systems
• Take a Model Driven Architecture approach
• SLAng semantics provide a reference for the behaviour of SLA aware systems.
• Use semantics led transformations to refine designs.
• Why not model the TAPAS architecture too?
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Conclusions
• We have formally defined the SLAng semantics for ASP SLAs.
• We embedded a model of the language into a model of services.
• We used constraints to show the effect that SLAs have.
• The semantic provides a basis for comparing SLAs – compatibility.
• The semantic forms the basis for a UML extension allowing analysis and implementation activities.
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Future Work
• Starting to be overwhelmed by UML extensions.
• Lots of confusing models. Are they models, meta-models, domain models, semantic models, domain specific languages, UML extensions?
• Need a unified approach to handling diverse model information, probably based on meta-modelling and consistency rules.